Bright Rings Found Around Sunspots

BOULDER--The Chinese noticed dark spots on the sun as early as 25 B.C.and Galileo gazed at them through his telescope in 1611, but they haveremained cloaked in mystery over the centuries. Now researchers at theNational Center for Atmospheric Research (NCAR) have found bright ringsaround eight sunspots. The presence of these rings sheds light on whysunspots are dark, and it could spell trouble for conventional models ofturbulent diffusion. The research is published this week in the journalNature. NCAR's primary sponsor is the National Science Foundation.

Scientists have been searching for the rings since they were firstpredicted 25 years ago. According to lead author Mark Rast, the rings'presence supports the idea that the spots appear dark because theirmagnetic fields block heat transport. "These findings could change ourunderstanding of how heat is transported under turbulent conditions," hesays.

With temperatures of 4000 Kelvin (3700 degrees Celsius), sunspots areboth cooler and darker than the surrounding solar disk, or photosphere,which hovers at 6000 K (5700 degrees C) when the sun is quiet. The newlydiscovered rings are only 1% brighter than the quiet photosphere, andthey compensate for only 10% of the sunspots' missing energy. Theircontribution to the amount of solar energy reaching the earth isnegligible. But their existence could be earthshaking for turbulentdiffusion models. Evidence of even faint "bright rings" suggests thatconvective heat transport around sunspots is structured and vigorousrather than evenly diffuse, as the models indicate.

"The rings suggests that either sunspots are shallow phenomena," saysRast, "or else convective flows around the spots transport heat to thesurface more efficiently than the turbulent diffusion models suggest. Ibelieve the latter is the missing component in the models." Such flowsmay play an important role in sunspot birth and growth.

Scientists have long believed that sunspots are cross-sections ofmagnetic, rope-like structures whose origins lie deep in the sun'sinterior. Their missing heat, they say, should appear on the sun'ssurface as a bright ring around the spot. However, the rings have neverbeen conclusively observed. Current models explain their absence as theresult of heat dispersal through homogeneous turbulence in the sun'sinterior.

In the past, measurement of the rings has been difficult becausevertical magnetic flux tubes show up as bright splotches aroundsunspots, obscuring the faint rings. With the aid of the Precision SolarPhotometric Telescope at NCAR's Mauna Loa Solar Observatory in Hawaii,Rast found rings around eight spots in all solar spectrum wavelengthsmeasured. According to Rast, the bigger the spot, the hotter andbrighter the ring. He then analyzed data taken with NCAR's AdvancedStokes Polarimeter at the National Solar Observatory in New Mexico,which measures the sun's magnetic field. His analysis showed that only asmall fraction of the rings' brightness was due to magnetic intensity ofvertical flux tubes. The rest, he believes, is heat suppressed by themagnetic field within the spot, which then emerges as a subtle halo.

The Precision Solar Photometric Telescope was developed at the NationalSolar Observatory as part of NSF's Radiative Inputs from Sun to Earth(RISE) program, which is dedicated to understanding solar radiativevariability as a possible driver of climate change. NCAR is managed bythe University Corporation for Atmospheric Research, a consortium ofmore than 60 universities offering Ph.D.s in atmospheric and relatedsciences.

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Visuals: Images are available atftp://ftp.ucar.edu/communications/Rast1099.

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